This invention relates to wire obtained by drawing high-carbon steel after patenting and to a method of producing the wire. More particularly, this invention relates to wire rod used for ACSR (Aluminum Conductor Steel Reinforced Wire)for reinforcement of aluminum power-transmission lines, elevator cable, rope wire, galvanized steel wire and the like, i.e., wire rod that is a product made by drawing, without additional processing, after controlled-cooling following hot rolling and a method of producing the same, steel wire obtained by subjecting a wire rod after hot rolling to draw processing including intermediate patenting, and small-diameter steel wire of high fatigue strength for use in steel cord, hose wire, bead wire, control cables, cut wire, saw wire, fishing line and the like, and to a method of producing the same.
In the case of wire made from 0.6% or higher high-carbon steel for use in rope and the like, the product wire rod is generally produced by hot rolling and processing the steel to obtain a wire rod of 5.0-16 mm diameter whose microstructure is thereafter adjusted by controlled cooling. Such wire rods are either made into wire, without additional processing, by drawing for regulation of wire diameter and mechanical properties or made into wire by drawing conducted after another adjustment of microstructure by an intermediate patenting treatment such as lead patenting conducted before drawing or in the course of drawing. Such wires are twisted into rope. They may, as required, be used improving of corrosion resistance by hot-dip galvanizing before drawing or in the course of drawing. On the other hand, small-diameter wire rod used for steel cord and the like is subjected to drawing and intermediate patenting treatment and is then processed into still finer wire of 1.0-2.2 mm diameter. This wire is subjected to final patenting to obtain a pearlite steel wire. Then, after plating, e.g., brass plating, it is processed into 0.15-0.35 mm diameter filament by drawing using drawing dies.
The wire rods used in ropes etc. discussed in the foregoing are desired to exhibit various characteristics, including high strength, excellent drawability and excellent fatigue property. The filaments used in steel cord etc. are made into steel cords with various twist configurations matched to the use conditions. In addition to the various characteristics mentioned above, such twisted steel wires are also desired to have excellent twisting properties.
High-quality wire rods for steel wire, and steel wire, have therefore been developed in response to the foregoing requirements. For example, JP-A-(unexamined published Japanese patent application)60-204865 teaches an ultra-fine wire and carbon steel wire rod for steel cord exhibiting properties of little wire breakage during twisting, high strength, high toughness and high ductility obtained by controlling the Mn content to less than 0.3% to suppress occurrence of overcooled texture after lead patenting and controlling the content of elements such as C, S and Mn. On the other hand, JP-A-63-24046 teaches a wire rod for high-toughness, high-ductility, ultra-fine wire wherein the Si content is set at not less than 1.00% to increase the tensile strength and thereby reduce the drawing rate of the lead patented steel. While these techniques may be able to achieve high strength, they are, however, incapable of providing sufficient fatigue strength. Further, JP-A-63-241136 teaches a method of improving the fatigue strength of steel wire obtained by drawing by adjusting the steel wire microstructure to an upper bainite structure throughout. Since the entire wire microstructure is given bainite, however, this technique is in actuality applied only to patented wire of a diameter of not greater than 1.5 mm. As none of the foregoing technologies achieve both high strength and high fatigue-strength, the development of a steel wire of high strength having high fatigue strength is desired.
The present invention was accomplished in light of the aforesaid current state of the art and, as its purpose, provides a wire rod enabling production of steel wire having unprecedented high fatigue strength at high strength and an ultra-fine wire having high fatigue strength at high strength for use in reinforcing rubber, tires and the like. The gist of the present invention is as follows:
(1) A wire rod for high fatigue-strength steel wire characterized in being a steel containing, in mass %, 0.6-1.3% of C and having a steel microstructure wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(2) A wire rod for high fatigue-strength steel wire characterized in being a steel containing, in mass %, 0.6-1.3% of C, 0.1-1.5% of Si and 0.2-1.5% of Mn, the balance being substantially iron and unavoidable impurities, and having a steel microstructure produced by controlled cooling following hot rolling wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(3) A wire rod for high fatigue-strength steel wire set out in (2) above, characterized in further containing, as a steel component, in mass %, 0.05-1.2.% of Cr.
(4) A wire rod for high fatigue-strength steel wire set out in (2) or (3) above, characterized in further containing as a steel component, in mass %, 0.005-0.1% of V.
(5) A wire rod for high fatigue-strength steel wire set out in any of (2) to (4) above, characterized in further containing as steel component(s), in mass %, one or more of 0.005-0.1% of Al, 0.002-0.1% of Ti and 0.0005-0.01% of B.
(6) A wire rod for high fatigue-strength steel wire set out in any of (2) to (5) above, characterized in further containing as a steel component, in mass %, 0.05-1.0% of Ni.
(7) A wire rod for high fatigue-strength steel wire set out in any of (2) to (6) above, characterized in further containing as a steel component, in mass %, 0.05-1.0% of Cu.
(8) A wire rod for high fatigue-strength steel wire set out in any of (2) to (7) above, characterized in further containing as a steel component, in mass %, 0.001-0.1% of Nb.
(9) A high fatigue-strength steel wire characterized in being obtained by drawing a wire rod set out in any of (1) to (8) above.
(10) A drawn high fatigue-strength steel wire characterized in having a steel composition set out in any of (1) to (8 ) above and having a steel microstructure wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(11) A high fatigue-strength steel wire obtained by drawing a wire rod or a heat-treated wire, characterized in having a steel composition set out in any of (1) to (8) above and having a steel microstructure wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(12) A method of producing a high fatigue-strength steel wire characterized in working under a true strain of not less than 1, preferably not less than 2, a wire rod or heat-treated wire characterized in having a steel composition set out in any of (1) to (8) above and having a steel microstructure wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(13) A method of producing a drawn wire rod for high fatigue-strength steel wire characterized in hot-rolling a billet containing the steel components set out in any of (1) to (8) above into a wire rod of 5-16 mm diameter, next immersing the wire rod from austenite temperature region in a fused-salt bath of a temperature not lower than 450xc2x0 C. and not higher than 55xc2x0 C. and then in succession completing transformation in a fused-salt bath of not lower than 500xc2x0 C. and not higher than 600xc2x0 C. to obtain a steel microstructure wherein the area fraction of upper bainite measured in a cross-section thereof is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite.
(14) A method of producing a high fatigue-strength steel wire characterized in hot-rolling a billet containing the steel components set out in any of (1) to (8) above into a wire rod of 5-16 mm diameter, drawing and patenting the wire rod to obtain a wire of 0.8-2.8 mm diameter, thereafter heating the wire to not lower than 800xc2x0 C. to transform to an austenite, quenching it to a temperature range of 500-560xc2x0 C. for conducting isothermal transformation, thereby adjusting it to a steel microstructure wherein the area fraction of upper bainite is not less than 5% and not greater than 50%, the remainder being substantially composed of pearlite, and then after brass plating drawing it to a wire of 0.05-1.0 mm diameter.